What crucial chemical contribution resulted from the explosive end of the first stars?

Scattering the first synthesized heavy elements into the intergalactic medium.

The brief but energetic lives of the Population III stars culminated in pair-instability supernovae, which served as the universe's first major chemical enrichment event. During their core fusion, these massive stars synthesized heavier elements such as carbon, oxygen, and iron. When they exploded completely, these newly forged elements were violently ejected and mixed with the original, pristine cloud of hydrogen and helium. This created the necessary chemical feedstock—the building blocks—that allowed subsequent generations of stars (Population II and I) to form with the capability of creating planets and supporting complex chemistry.